If you don't know how to clean up junk files from Android phones so as to free up more space on your phone and optimize the performance, please don't worry, you can use Android data cleaner app. It helps you clear junk files from Android device in one click.http://www.android-data-eraser.com/resources/android-junk-files-cleaning.html

First you may download trusted app from the store that can clear junks. but be sure to read more on how to use the app correctly and to avoid spam.

Regarding slow phone, maybe you run lots of things at the same time, or you have adware on your device. Also some good apps may show clickable link to adware app like (wholesale clothing, car, flowers, roses, software....ets) make sure to click only on trusted page.